1. Field of the Invention
The present invention relates to a dual phase steel sheet with good bake-hardening properties and, more particularly, to a dual phase steel sheet having well-balanced strength and forming properties. This steel sheet has not only good bake-hardening properties but also good resistance to natural aging. (The term “bake-hardening properties” implies that the steel sheet improves in strength upon paint baking. The term “resistance to natural aging” implies that the steel sheet retains its characteristic properties (such as forming properties) without deterioration after aging at room temperature). The dual phase steel sheet according to the present invention will be widely used in automotive, electric, and machine industries and other industrial fields. The following description is mainly concerned with its use in automotive bodies as a typical example.
2. Description of the Related Art
There has been an increasing demand for steel sheets for automotive use which are thinner (for improved fuel consumption) and stronger (for improved collision safety) than before. Such steel sheets are required to exhibit good forming properties at the time of forming, such as press working. Unfortunately, improved strength often has an adverse effect on forming properties. Steel sheets for automotive use, which undergo complicated forming, are required to be comparatively soft (for easy forming) at the time of press working and to become highly strong at the time of heat treatment to bake the coating thereon which follows press working.
The above-mentioned bake-hardening is due to strain aging that occurs at a high temperature (about 150-200° C.) for paint baking. Strain aging results from interstitial elements (C and N) fixing dislocations. Therefore, paint baking offers the advantage of imparting high strength to the final product.
Incidentally, strain aging occurs also at normal temperature, and in this case, dissolved carbon and nitrogen in the steel migrate to fix dislocations even before paint baking. Any steel sheet with strain aging at normal temperature is poor in ductility due to yield elongation, and poor ductility leads to flaws (such as wrinkles) at the time of press working.
Consequently, automotive steel sheets are required to readily undergo strain aging at high temperatures for paint baking, thereby increasing in strength, and hardly undergo strain aging at normal temperature. In other words, they are required to be good in bake-hardening and also in resistance to natural aging.
Under these circumstances, there have been proposed steel sheets with improved bake-hardening, such as BH steel of quasi-IF (Interstitial Free) type. It contains about 30 ppm of dissolved carbon in the ferrite structure, so that dissolved carbon fix dislocations, thereby improving the bake-hardening properties. It is used mainly for the outer panel of automobiles.
Unfortunately, the BH steel of quasi-IF type mentioned above has a strength of about 440 MPa at most even after bake-hardening on account of its low content of dissolved carbon.
There is a kind of DP steel (Dual Phase Steel) which contains dislocations introduced into the parent phase ferrite by martensitic transformation. It has a low value of yield point as such but has a high value of yield point due to hardening after paint baking which fixes the above-mentioned dislocations and other dislocations introduced by working.
Moreover, there is a kind of so-called TRIP steel which is designed to improve the bake-hardening properties. TRIP steel is a steel which contains retained austenite of several to tens of percent in the metal structure, so that it exhibits high toughness after plastic forming. For example, Japanese Patent Laid-open No. 11565/2001 discloses a technology for increasing the amount of bake-hardening. This technology aims at developing a steel sheet that absorbs a large amount of collision energy to meet both requirements for safety of passenger cars and weight reduction of car body.
Generally, a conceivable mechanism which makes TRIP steel improve in bake-hardening is the bonding of carbon which originally exists in the ferrite to dislocations induced by working, as in the case of above-mentioned dual phase steel. This conception, however, does not explain why the steel increases in strength by 50 MPa or more by bake-hardening. Another conceivable mechanism has been proposed as follows. Retained austenite is transformed into martensite by plastic forming before bake-hardening. Carbon in the martensite releases itself at the time of paint baking. This carbon bonds to the dislocations in ferrite which have been introduced during working. In this way, hardening takes place.
Improvement in TRIP steel which inherently has well-balanced strength and workability has been made to provide a new steel sheet capable of high bake-hardening at the time of paint baking, as mentioned above. However, a steel sheet with high bake-hardening poses problems with increased yield point, decreased elongation, and aging namely deterioration with time in characteristic properties. These phenomena could possibly occur as follows. First, dislocations form from skin pass rolling or martensitic transformation during production, and then these dislocations catch carbon which has diffused and migrated from retained austenite after its decomposition that takes place for one reason or another, since TRIP steel contains retained austenite with a large amount of dissolved carbon. As the result, deterioration in characteristic properties such as increased yield point and decrease of elongation occurs. Such a steel sheet exhibits good workability immediately after production but deteriorates with time due to aging when it is worked by the user. Japanese Patent Laid-open No. 297350/2000 proposes an idea that a steel sheet is improved in bake-hardening properties and resistance to natural aging when it has the dual phase structure in which the principal phase is ferrite and the second phase is at least one of pearlite, bainite, martensite, and retained austenite, with dissolved nitrogen controlled in amount and positions where it exists. However, there seems to be room for further improvement in elongation.